Nobody knows. Or, if anyone knows, they won't say until it's announced at
https://blog.roku.com/By the way, that's always the answer for any questions that go "Does anybody know when Roku will ..." Always.
Now, we can tell you how to get Cinemax via Roku. If you subscribe to Sling TV, DirecTV Now, PlayStation Vue, or Hulu, you can add Cinemax to your service. Prices vary from $5/month to $15/month.
And, you can subscribe to Cinemax as a stand-alone service via PlayStation Vue; that is, without an existing service package.
Basil
https://www.basilsblog.com/
Roku Ultra (4660)
Apple TV (5th gen), TiVo
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Roku boxes from every generation.
Apple TV (2nd, 3rd, 4th gen)